1. Field of the Invention
The present invention relates to a pilot signal cancelling system that eliminates pilot signals mixed in composite signals in a stereo modulation. More specifically, this invention relates to the pilot signal cancelling system in which the signal-to-noise ratio does not deteriorate.
1. Description of the Prior Art
FIG. 1 shows the configuration of a conventional pilot signal cancelling system. Referring to FIG. 1, the pilot signal cancelling system has the following functions and construction.
First, a composite signal is inputted from an input terminal 1. Next, a pilot signal of 19 kHz is level-detected by a level detector 3. Then, a lamp drive circuit 9 is activated by a lamp trigger circuit 7 with a voltage smoothed by a low-pass filter (LPF) 5. An output of LPF 5 flows through a voltage control amplifier (VCA) 11 and a cancelling wave generating circuit 13 to generate a pilot cancelling wave. The pilot cancelling wave is amplified by a first amplifier pilot cancelling wave is subtracted from the pilot signal in the composite signal by an adder 19, then inputted into the level detector 3. In other words, a negative feedback loop is constituted by the output of LPF 5, VCA 11, cancelling wave generating circuit 13 and first amplifier 15, in this order. Accordingly, as an output of the cancelling wave generating circuit 13 there is generated a cancelling wave proportional to the pilot signal level in the composite signal. Furthermore, the output of cancelling wave generating circuit 13 is amplified by a second amplifier 17. The amplified output of cancelling wave generating circuit 13 is subtracted from the original pilot signal by the adder 21. As a result, a signal in which the pilot signal has been cancelled is obtained in a composite signal output terminal 23.
Since the output of cancelling wave generating circuit 13 is proportional to the original pilot signal level in the composite signal, a signal (without pilot signal) in response to the original pilot signal is obtained in a composite signal output terminal 23.
The VCA 11 is driven by the output of LPF 5. Thus, a leak current from LPF 5 causes the VCA 11 and the cancelling wave generating circuit 13 to generate a slight amount of cancelling wave even in a monaural mode in a conventional system. This is a malfunction of the system since the monaural signals, do not contain any pilot signals thus, requiring no activation of the cancelling wave generating circuit 13. Such an unnecessary cancelling wave is outputted to the output terminal 23 through the second amplifier 17 and an adder 21. Consequently, an S-N ratio is deteriorated by the cancelling wave in a monaural mode. In particular, it creates a problem when the leak current increases with a high temperature.
Even when the positive signal state of (a pilot signal) is abruptly changed to the no pilot signal stated in stereo broadcasting, the cancelling wave is outputted, causing the S-N ratio to deteriorate. This is because a voltage which drives VCA 11 is still ON until the VCA driving voltage reaches to OFF level, due to a time constant of LPF 5 even though the lamp trigger circuit 7 is off.
The above problem becomes more serious when the composite signals are switched at high speed. For example, when another station is searched intermittently at high speed while listening to a station program, the cancelling wave alone is outputted without the presence of any original pilot signal during the search, thus deteriorating the S-N ratio. This problem is best illustrated with reference to the following figures.
FIG. 2 shows an input/output wave form of the pilot cancelling system when a stereo program (having a pilot signal) is repeatedly switched over to the other station's programs (no pilot signal), and vice versa. FIG. 2B shows a pilot signal wave form in the composite signal at the input terminal 1. FIG. 2C shows an output wave form of LPF 5. FIG. 2D shows an output wave form of cancelling wave generating circuit 13 and second amplifier 17. FIG. 2E shows a signal, in which the pilot signal has been cancelled, at the output terminal 23.
When the state of the pilot signal is switched from having a pilot signal to no pilot signal, the output of LPF 5 decreases gradually with the time constant and is discharged for the duration of --a--indicated in FIG. 2C. The cancelling wave generating circuit 13 and the second amplifier are still outputting the cancelling wave form for the duration of --b--(shown in FIG. 2E) until the voltage reaches to the OFF level as shown in FIG. 2C. Therefore, the composite signal at output terminal 23, which is the difference between the pilot waves shown in FIG. 2B and FIG. 2D is shown in FIG. 2E. FIG. 2E shows that even if there is no pilot signal in range b, the cancelling wave is outputted, causing the S-N ratio to deteriorate.
As shown above, there are shortcomings in the conventional pilot cancelling system. Namely, the S-N ratio is deteriorated due to the leak current in the monaural broadcasting and the time constant of low pass filter 5 at the time of switching the pilot signal on and off.